In a three-phase high-voltage converter mounted in, for example, a fuel cell hybrid vehicle, when currents flowing through three phases reach a peak at the same timing, an excessive current flows through circuits. Thus, the three-phase high-voltage converter is controlled so as to set the phase difference between the currents flowing through the respective three phases to 2π/3 in order to reduce circuit loads.
As a conventional technique in which, in the control of an N-phase converter, the phases of currents flowing through the respective phases are shifted from one another by 2π/N, for example, Patent Document 1 discloses a multiphase multiple chopper apparatus including (m) chopper devices connected together in parallel. In the multi-phase multiple chopper apparatus, each chopper device is responsible for 1/m of the total current supplied to an output load, and the phases of the currents each flowing between the chopper devices are shifted from one another by 2π/m. Furthermore, Patent Document 2 discloses a voltage raising and lowering converter providing the function of raising a lower voltage by a factor of (n) to obtain a higher voltage and the function of lowering the higher voltage by a factor of 1/n to obtain the lower voltage. In the voltage raising and lowering converter, the number of phases is an integral multiple N of an integer N′ closest to (n), and N voltage raising and lowering choppers connected together in parallel are combined together to form an N-phase N-multiple circuit. In this case, currents flowing through the respective phases are controlled to be shifted from one another by 2π/N.
An output voltage from the multiphase converter as described above and currents flowing through circuits in the converter are generally controlled by using pulse width modulation (PWM) signals to controllably power on and off semiconductor switching elements such as IGBTs (Insulated Gate Bipolar Transistors) included in the voltage raising and lowering circuit in the multiphase converter. For example, there has been disclosed a technique which involves, in controlling a three-phase high-voltage converter, setting the phase difference between currents flowing through the respective phases by performing control such that the phase difference between PWM signals allowing driving of the respective semiconductor switching elements for three phases is set to 2π/3.
Furthermore, in the control of the three-phase high-voltage converter, the carrier period of each PWM signal may need to be changed while the converter is in operation. For example, if the duty ratio of the PWM signal remains high (for example, at least 80%) for at least a given time, the semiconductor switching elements may disadvantageously generate heat as a result of heat loss. With the duty ratio of the PWM signal constant, an increase in the carrier period of the PWM signal reduces the heat generated by the semiconductor switching elements. Thus, in the control of the three-phase high-voltage converter, if the duty ratio of the PWM signal is expected to remain high for at least a given time in connection with, for example, a target output voltage or if the temperature of the IGBTs increases, the carrier period may be controlled to be changed to a value larger than the current one in order to reduce the heat generated by the semiconductor switching elements.
The generation of PWM signals and the control of period and phase of the PWM signal can be carried out using an arithmetic device such as a CPU (Central Processing Unit). For example, a carrier signal used to generate a PWM signal can be generated by repeating a process of allowing a timer counter of the CPU to start counting and initializing the counter value when the counter value reaches a preset maximum value. The carrier period is defined by the maximum counter value. Hence, the carrier value can be changed by changing the maximum value.